1. Department of Chemistry, Moscow State University, Leninskie gory, 119992 Moscow, Russia, Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin-Dahlem, Germany, Institute of Problems of Chemical Physics RAS, 142432 Chernogolovka, Moscow Region, Russia, and Institut für Festkörperphysik, Technische Universität Dresden, D-01062 Dresden, Germany